Silicon carbide, a crystalline compound of silicon and carbon, has long been known for its hardness, its strength and its excellent resistance to oxidation and corrosion. Silicon carbide has a low coefficient of expansion, good heat transfer properties and maintains high strength at elevated temperatures. In recent years, the art of producing high density silicon carbide bodies from silicon carbide powders has been developed. Methods include reaction sintering, hot pressing and pressureless sintering (initially forming the article and subsequently sintering under substantially pressureless conditions). The high density silicon carbide bodies so produced are excellent engineering materials and find utility in fabrication of components for turbines, heat exchange units, pumps, and other equipment or tools that are exposed to severe wear and/or operation under high temperature conditions.
The reaction sintering process, also called the reaction bonding process, is typically carried out by forming silicon carbide in situ by the reaction of silicon and carbon in a porous body of silicon carbide. In such processes, particulate silicon carbide is initially mixed with particulate carbon or a carbon source material. The mixture is then formed into a green body of the desired shape. Frequently, the initial mixture includes a binder, such as a thermosetting resin material, to add strength to the green body and also as a carbon source material. The green body may be baked at temperatures to set and pyrolize the binder. The shaped green body is impregnated, or infiltrated, typically by immersing the body in molten silicon at temperatures ranging from about 1400.degree. to about 2300.degree. C. The silicon reacts with the available carbon in the green body to form silicon carbide. Usually, the amount of carbon in the green body is less than the stoichiometric amount needed to combine with the added silicon. Thus, reaction sintered silicon carbide products typically contain from about 10 to about 25 percent by volume of free silicon.
Examples of articles of silicon and silicon carbide mixtures are described in U.S. Pat. Nos. 1,012,531; 1,030,327; 1,906,963; 2,242,482 and 3,459,842.